NF3 is used as a dry etching gas or the like in, for example, the production of semiconductor devices and, in general, the processes for producing NF3 are roughly classified into chemical processes and electrolytic processes.
Known examples of chemical processes include:
(1) a method of blowing F2 gas and NH3 gas into fused acidic ammonium fluoride (see, Japanese Examined Patent Publication No. 55-8926, JP-B-55-8926),
(2) a method of reacting a metal fluoride ammonium complex in a solid form with F2 gas (see, Japanese Examined Patent Publication No. 60-71503, JP-B-60-71503), and
(3) a method of directly reacting F2 gas with NH3 gas (see, Japanese Unexamined Patent Publication No. 2-255513, JP-A-2-255513).
On the other hand, known examples of the electrolytic process using fused acidic ammonium fluoride as an electrolyte include:
(4) a method of performing electrolysis using graphite as an anode, and
(5) a method of performing electrolysis using nickel as an anode.
In addition, Ruff et al. have reported that, using the chemical process of reacting F2 with NH3 in a gaseous phase, NF3 can be synthesized, though the yield is as low as 5% or less (see, Z. anorg. allg. Chem., 197, 395 (1931)). Also, Morrow et al. have reported that NF3 was synthesized similarly in a gaseous phase (see, J. Amer. Chem. Soc., 82, 5301 (1960)).
However, in the conventional direct fluorination process of synthesizing NF3 from NH3 as the reaction substrate using F2 gas, the F2 gas used is very highly reactive, therefore, explosion or corrosion may occur between the substrate and F2 gas. Furthermore, these reactions incur generation of a large amount of heat of reaction to increase the temperature in the reactor, as a result, the yield disadvantageously decreases due to decomposition of the product NF3 or due to the generation of N2, HF or NH4F by the side reactions. The method described in JP-A-2-255513 has a problem in that NF3 is produced using F2 gas in 3 to 20 times based on NH3, where the reactor temperature is kept at 80 to 250° C. within a heat medium, therefore, the yield based on the F2 is low and the profitability is low.
In the direct fluorination method using F2 gas, heat of reaction of about −110 kcal/mol is generated when one hydrogen in the substrate is displaced by one fluorine. Therefore, in the case of producing NF3 by reacting F2 with NH3, heat of reaction of about −330 kcal/mol is generated upon displacement of hydrogen with fluorine and this generation of heat readily causes breakage of the N—F bond or explosion and moreover decreases the yield, giving rise to problems in the production.